Frequency measuring apparatus and frequency measuring method

ABSTRACT

A frequency measuring apparatus for measuring the frequency of an input signal to be measured with high accuracy. The apparatus includes a crystal oscillator for generating an internal reference frequency, a temperature sensor provided on the crystal oscillator, a writable memory and an estimation control device for storing in the memory a correspondence relationship between an absolute frequency to be taken as a reference at a predetermined temperature measured by the temperature sensor and the internal reference frequency as error correction information and estimating the frequency ortho input signal in conformity with the internal reference frequency corrected by the error correction information upon the input signal.

BACKGROUND OF THE INVENTION

1. Field of the invention

The present invention relates to a frequency measuring apparatus formeasuring the frequency of a signal with high accuracy using a crystaloscillator as a reference frequency source, and more specifically to afrequency measuring apparatus and a frequency measuring method whereinthere are entirely eliminated the need of high-degree techniques andmuch labor in manufacture, adjustment, and inspection of conventionalcrystal oscillators and wherein the accuracy of the measurement isfurther improved, and further wherein the cost of manufacture of theapparatus is sharply reduced and the apparatus is miniaturized.

2. Description of the Prior Art

An apparatus for measuring the frequency of a signal generally requiresa reference frequency source. Such reference frequency sources includethe so-called temperature compensated crystal oscillator (TCXO) for theaccuracy of measurement up to ±1 ppm, and the so-called oven-controlledhighly stabilized crystal oscillator (OCXO) when further higher accuracyis required. These oscillators are manufactured as oscillator units bythose skilled in the art. Such an oscillator unit satisfies requirementsfor the reference frequency source as its unit.

More specifically, the requirements are such that an absolute frequencyis ensured at the center of a variable width, that an associatedfrequency is variable within a predetermined width with use of avariable capacitor in order to moderate a change with the lapse of timein the associated frequency, and that a change in an output frequencyfalls within a predetermined deviation with respect to environmentalchanges such as changes with the lapse of time, and in temperature and apower supply.

Manufacturers in this field conventionally design, manufacture, inspect,and ship such oscillators so as to satisfy these requirements. In otherwords, TCXO and OCXO as conventional reference oscillators require ahigh-degree technique and labor which can not be dealt with by personsother than those skilled in the art to result in the high cost where acost rate of a reference oscillator to that of a frequency measuringapparatus including the reference oscillator is increased.

The following is an example of the aforementioned high-degree techniqueand labor upon manufacturing the Tcxo and Ocxo.

In a required temperature range there is necessary a monotonicallydecreasing temperature characteristic for a frequency beforecompensation, so that the frequency is needed to be variable over ±15ppm in a temperature range of from 0° to 70° C. for example. In additionto this requirement a frequency variation of about ±5 ppm using avariable capacitor, etc., is required for moderating a change with thelapse of time, and further a function to moderate manufacturingtolerance of a crystal oscillator, say ±10 ppm is required.Possibilities of so much frequency variations mean that there must alsoadditionally be taken into consideration frequency variations caused bychanges with the lapse of time and temperature changes of circuitcomponents constituting a reactance network for varying an associatedfrequency.

This in turn causes severe variations of load capacitance viewed from acrystal oscillator which further cause a change in the conditions of anoscillator loop and result in variations of oscillation redundancy andof an output level.

The variations of load capacitance to moderate the change with the lapseof time in an oscillation frequency are equivalent to shifting of theangle of cutting of a crystal and result in a change in the temperaturecompensation characteristic.

There is further a limitation to the crystal oscillator that acapacitance ratio (a ratio between parallel capacitance and equivalentseries resonance circuit capacitance) of the quartz oscillator should belower, in order for the crystal oscillator to have a required frequencyvarying function.

The conventional technique suffers from a difficulty of its not ensuringstable oscillation unless the aforementioned complicated problems aresolved.

The conventional technique further suffers from a difficulty that sincea frequency measuring apparatus and an oscillator are manufactured bydifferent manufacturers both manufacturers do not agree with each otherin view of the times for deliveries and the prices thereof, and areobliged to wasteful time and negotiations.

The conventional technique furthermore suffers from a difficulty of itsrequiring a large-sized mechanical structure in order to satisfynecessary requirements and specifications and hence of its beinghindered from realizing a compact frequency measuring apparatus.

A concrete example of a conventional OCXO (oven-controlled highlystabilized crystal oscillator) will be demonstrated in the following fordescription of the difficulties with the same.

FIG. 11 is a block diagram illustrating an example of the constructionof a general function of a conventional OCXO.

In the same figure, numeral 201 is an oven control unit by which aregulated temperature oven 202 is controlled in its temperature to aregulated predetermined temperature. The regulated temperature oven 202contains part or the whole of an oscillator output unit 203, a crystaloscillator 204, a frequency adjustment unit 205, and part or the wholeof a frequency varying unit 206, all of these units constitute anoscillation loop.

One circuit may commonly use part or the whole of the frequencyadjustment unit 205 and the frequency varying unit 206. The regulatedtemperature oven control unit generally keeps accurately and with highstability regulated temperature oven internal temperature at a minimumpoint temperature of a frequency-temperature characteristic of a crystaloscillator used in the present oscillator, i.e., a turning pointtemperature on the high temperature side when the characteristic is acubic curve.

For example, the regulated temperature oven internal temperature shouldbe kept within ±0.1° C. with respect to the minimum point temperature,with its variations kept within 0.01° C. The oscillation/output unit 203supplies a crystal oscillator with energy required for the crystaloscillator to vibrate in a predetermined mode and outputs oscillationfrequency of the crystal oscillator to the outside. For the crystaloscillator 204 there are used those having high stability and lessvariations with the lapse of time, and the crystal oscillator 204 isdesigned and manufactured such that the minimum point temperature of thefrequency-temperature. characteristic falls within a predeterminedtemperature range, say, within a range of from 70° C. to 80° C.

The frequency adjustment unit 205 is a circuit for adjusting a variationamount by the frequency varying unit 206 such that an oscillator outputat the state of zero of the variation amount falls within apredetermined deviation with respect to a nominal frequency of theoscillation frequency, say, within a range of ±0.5 ppm of the nominalfrequency, for the purpose of correction of manufacturing deviation ofthe foregoing crystal oscillator and other circuit components. Thefrequency varying unit 206 is a circuit for correcting the change withthe lapse of time of the crystal oscillator, for which a variable widthof ±0.1 ppm˜1 ppm is typically required. It should herein be noticedthat the nominal frequency is a reference absolute frequency to beoutput from the oscillator, i.e., an ideal value for which a certainallowable error range is specified to actual oscillators.

The regulated temperature oven highly stabilized crystal oscillatorconstructed as above has a difficulty of its being very expensivebecause a highly accurate adjustment is required upon its manufacture asdescribed in detail below.

In order to manufacture the crystal oscillator such that its frequencyat the minimum point temperature falls within a predetermined deviation,very highly accurate and complicated. adjustment process is firstnecessary. More specifically, although for the crystal oscillator 204 anelectrode thereof is formed by deposition of a metal film, platetemperature is different from the minimum point temperature because thecrystal plate is overheated by the deposition at that time.

Accordingly, a work to adjust the minimum point temperature of thecrystal oscillator and the frequency of the same to desired valuesdepositing the metal film requires very high-degree technique and muchlabor but with the reduced yield.

Frequency adjustment after assembly of the oscillator is alsotroublesome. Although the oscillator is generally adjusted to a desiredfrequency at the temperature in the thermostatic oven while replacing oradding reactive parts, i.e., a capacitor to the oscillator, there mustbe repeatedly executed a work to take out an oscillation circuitcomponent assembled in the thermostatic oven for each adjustment andadjust it at different temperature from the former situation, and againassemble the component into the thermostatic oven and thereafter returnthe temperature to the predetermined temperature putting many hours foradjustment of the frequency of the oscillator.

It is further required to check that a change in the load capacity ofthe oscillation circuit resulted in by adjusting the frequencyadjustment unit to correct the manufacturing deviation of the crystaloscillator does not affect the variation performance like an originaldesign. Provided the variation performance is affected by the change inthe load capacity of the oscillation circuit, the aforementionedfrequency adjustment work is again required after replacing the parts ofthe frequency variation unit.

Even in the thermostatic oven control function, there is furtherrequired a work of the minimum point temperature matching.

More specifically, since the minimum point temperature of the crystaloscillator depends on the cutting angle of the crystal oscillator plate,the minimum point temperature is impossible to fall within ±0.1° C. inview of the limitation of processing accuracy.

Measuring apparatuses such as highly accurate frequency measuringapparatuses and spectrum analyzers require a highly accurate referencefrequency signal and conventionally use the aforementioned OCXO output.

The OCXO used for this uses a signal frequency itself output therefromas a reference signal, so that the absolute value of that frequency mustbe coincide with a nominal frequency with high accuracy to result in thevery high cost of the OCXO occupied in the whole measuring apparatus.

SUMMARY OF THE INVENTION

In order to solve the problems with the prior art, it is an object ofthe present invention to provide a frequency measuring apparatus and afrequency measuring method wherein high-degree techniques and labor uponmanufacturing, adjusting, and inspecting a conventional crystaloscillator used as a reference frequency source are completelyeliminated as described above and wherein accuracy of frequencymeasurement is further improved and the cost of manufacturing theapparatus is sharply reduced together with insurance of miniaturizationof the apparatus.

It is another object of the present invention to provide a frequencymeasuring apparatus and a frequency measuring method using an OCXOwherein minimum point temperature of a frequency-temperaturecharacteristic of a crystal oscillator in the OCXO is kept unchangedwith ease and wherein a deviation between an actual oscillationfrequency at the minimum point temperature and nominal frequency isremoved.

To achieve the above object, a frequency measuring apparatus formeasuring the frequency of an input signal to be measured with highaccuracy according to the present invention comprises a crystaloscillator for generating an internal reference frequency, a temperaturesensor provided on said crystal oscillator, writable memory means, andestimation control means for storing as error correction information acorrespondence relation between absolute frequency to be taken as areference at a predetermined temperature measured with said temperaturesensor and the internal reference frequency from said crystal oscillatorand estimating the frequency of said input signal upon said input signalbeing input in conformity with the internal reference frequencycorrected by said error correction information.

A frequency measuring method of measuring the frequency of an inputsignal to be measured according to the present invention comprises thesteps of measuring a correspondence relation between said internalreference frequency and the reference absolute frequency at a pluralityof temperature measurement points of a crystal oscillator for generatingthe internal reference frequency, storing said measured correspondencerelation in memory means as error correction information, measuring,when said signal to be measured being input, the temperature of saidcrystal oscillator at that time, reading said error correctioninformation at said measured temperature from said memory means, andcorrecting said internal reference frequency based upon said read errorcorrection information and estimating the frequency of said input signalto be measured in conformity with the corrected internal referencefrequency.

A frequency measuring apparatus for measuring the frequency of an inputsignal to be measured with high accuracy comprises a highly stablecrystal oscillator (OCXO) using a thermostatic oven for generating aninternal reference frequency, a temperature sensor for measuringtemperature of the thermostatic oven of the OCXO, writable memory means,and estimation control means for executing various estimation control,said estimation control means serving to obtain the temperature of thethermostatic oven measured with said temperature sensor and minimumpoint information of the crystal oscillator of the OCXO from theinternal reference frequency at said temperature and storing them insaid memory means, control said OCXO such that said OCXO maintain theminimum point temperature in conformity with the minimum pointtemperature information from said memory means, and obtain as errorcorrection information a correspondence relation between an absolutefrequency at said minimum point temperature to be taken as a referenceand the internal reference frequency from the OCXO and store it in saidmemory means, and further estimate upon said signal to be measured beinginput the frequency of the same signal in conformity with the internalreference frequency corrected by said error correction information.

A frequency measuring method of measuring the frequency an input signalto be measured with high accuracy comprises the steps of: measuring aninternal reference frequency at a plurality of temperature measurementpoints of a highly stable crystal oscillator (OCXO) using a thermostaticoven for generating said internal reference frequency, estimatingminimum point temperature from the information of the internal referencefrequency at said plurality of the temperature measurement points,storing the estimated minimum point temperature in memory means, keepingthe temperature of the thermostatic oven of the OCXO at said minimumpoint temperature in conformity with the minimum point temperature fromsaid memory means, measuring a correspondence relation between theabsolute frequency to be taken as a reference at said minimum pointtemperature and the internal reference frequency from the OCXO, storingsaid measured correspondence relation in the memory means as errorcorrection information, reading said error correction information fromsaid memory means upon said signal to be measured being input, andcorrecting said internal reference frequency based on the read errorcorrection information and estimating the frequency of the input signalto be measured in conformity with the corrected internal referencefrequency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a first embodiment of a frequencymeasuring apparatus according to the present invention;

FIG. 2 is an internal block diagram of an OCXO illustrated in FIG. 1;

FIG. 3 is a flow chart illustrating the entire operation of thefrequency measuring apparatus illustrated in FIG. 1;

FIG. 4 is a flow chart illustrating a process of obtaining errorcorrection information illustrated in FIG. 3;

FIG. 5 is a flow chart illustrating a process of measuring a frequencyto be measured illustrated in FIG. 3;

FIG. 6 is a block diagram illustrating a first modified example of anembodiment of the frequency measuring apparatus illustrated in FIG. 1;

FIG. 7 is a circuit diagram of a temperature sensor in the firstmodified example of the embodiment of the frequency measuring apparatusillustrated in FIG. 1;

FIG. 8 is a perspective view illustrating a packaging structure for theOCXO and a temperature sensor in the frequency-measuring apparatusillustrated in FIG. 1;

FIG. 9 is a block diagram of a portion of a second embodiment of thefrequency measuring apparatus according to the present invention;

FIG. 10 is an internal block diagram of the OCXO illustrated in FIG. 9;and

FIG. 11 is a block diagram of a conventional OCXO.

DESCRIPTION OF PREFERRED EMBODIMENTS

In what follows, the present invention will be described on the basis ofillustrated embodiments. FIG. 1 is a block diagram of a first embodimentof a frequency measuring apparatus according to the present invention.

As illustrated in FIG. 1, the frequency measuring apparatus comprises afirst input terminal 1 into which a frequency f x to be measured isinput, an input circuit 2 connected to the first input terminal 1, apre-scaler 3 connected to the input circuit 2, a highly stable crystaloscillator (OCXO) 4, a temperature sensor 5 provided on the OCXO 4, asecond input terminal 6 into which an absolute frequency f_(ro) to betaken as a reference is input, a gate circuit 7 connected to thepre-scaler 3, the second input terminal 6, and the OCXO 4, first andsecond counters 8, 9 connected to the gate circuit 7, a non-volatilememory 10, an output display 11, and a pre-scaler 12 connected to thetemperature sensor 5, the gate circuit 7, the second counter 9, thenon-volatile memory 10, and the output display 11.

As illustrated in FIG. 2, the OCXO 4 is constructed such that a crystaloscillator 15 and an oscillation/output unit 16 are disposed in athermostatic oven 14 forming an oscillation loop. The thermostatic oven14 includes the temperature sensor 5 provided thereon.

In the following, there will be described the operation of the frequencymeasuring apparatus constructed as above, i.e., a frequency measuringmethod.

The operation of the frequency measuring apparatus comprises, asschematically illustrated in the flow chart of FIG. 3, a process 100 ofobtaining error correction information wherein there is stored in thenon-volatile memory 10 a correspondence relation between the absolutefrequency f_(ro) to be taken as a reference at a plurality oftemperature measurement points within an operation temperature range inthe oven of the OCXO 4 and the internal reference frequency f_(r) fromthe OCXO 4 as the error correction information, and a process 101 ofmeasuring a frequency f_(x) to be measured wherein upon the frequencyf_(x) being input after the process 100 of obtaining the errorcorrection information temperature in the thermostatic oven at that timeis measured, and the internal reference frequency f_(r) from the OCXO 4is corrected on the basis of the error correction information from thenon-volatile memory 10, and further the frequency fx to be measured isestimated in conformity with the corrected internal reference frequencyand is output and displayed.

In the following, there will be described the process. 100 of obtainingthe error correction information with reference to the flow chart ofFIG. 4.

In step 103 in FIG. 4, once the absolute frequency f_(ro) (10 MHz in thepresent case) to be taken as a reference is first input from the secondinput terminal 6 as an external reference input terminal, the gatecircuit 7 is controlled by the processor 12 and the absolute frequencyf_(ro) is counted by the second counter 9. Simultaneously, in step 104,the internal reference frequency f_(r) (10 MHz) from the OCXO 4 iscounted by the first counter 8 on the basis of the control of theprocessor 12.

In step 105, the temperature in the thermostatic oven is measured by thetemperature sensor 5 when the first and second counters 8, 9 are in thecourse of counting, and it is judged by the processor 12 whether or notthe temperature in the thermostatic oven is at a measurement pointwithin the operation range.

Once the temperature in the thermostatic oven is judged to be at themeasurement point within the operation range in step 105, in step 106there is estimated a difference or a ratio between the count of thefirst counter 8 and the count of the second counter 9 at the temperaturemeasurement points in the thermostatic oven (i.e., a difference or aratio between the internal reference frequency f r and the absolutefrequency f_(ro)) and is stored in succession in the non-volatile memory10 in a table map format as the error correction information S. In step107, it is judged whether or not the error correction information S atall measurement points within the operation range has been estimated andif it has been estimated, the processing has been completed.

It is significant to process the resulting error correction informationS in order to speed up the processing at the actual frequencymeasurement. It is for example considered for the processing that atable of finer temperature sensor output-correction data is preparedusing a proper approximation technique or coefficients of an approximatepolynomial are previously estimated and are stored in the non-volatilememory 10.

In this case, the table of the finer temperature sensoroutput-correction data is prepared and is stored in the non-volatilememory 10.

In the following, there will be described the measuring process 101 ofthe frequency to be measured with reference to the flow chart of FIG. 5.

Once in step 109 in FIG. 5 the frequency f_(x) to be measured is inputfrom the first input terminal 1, it is divided by the pre-scaler 3through the input circuit 2. It should herein be understood that afrequency division ratio in the pre-scaler 3 is assumed to be p, thedivided frequency is f_(x) /p.

In step 110, the gate circuit 7 is controlled by the processor 12 andthe divided frequency f_(x) /p is counted by the first counter 8.Simultaneously, in step 11, the internal reference frequency f_(r) fromthe OCXO 4 is counted by the second counter 9 on the basis of thecontrol of the processor 12.

In step 112, the temperature in the thermostatic bath of the OCXO 4 bythe temperature sensor 5 is measured by the processor 12 in the courseof the counting by the first and second counters 8, 9 and the errorcorrection information S at the measurement point is read from thenon-volatile memory 10.

In step 113, the frequency f_(x) is estimated in the processor 12 basedon the internal reference frequency f_(r) corrected with the errorcorrection information S, and is sent to the output display 11 for itsdisplay.

The estimation of the frequency f_(x) will be described in detail.Provided the counts of the first and second counters 8, 9 are assumed tobe q and r without causing an overflow, holds.

    f.sub.x =p·q/r·f.sub.r                   (1)

The internal reference frequency f_(r) of the OCXO 4 has an initialdeviation at the ordinary temperature of 25° C. and a deviation causedby a temperature difference from the ordinary temperature with respectto the absolute frequency f_(ro), which should be originally taken as areference.

Provided these deviations are assumed to be the error correctioninformation S (ppm) holds,

    f.sub.ro =(1+S×10.sup.-6)f.sub.r                     (2)

and hence

    f.sub.x ={(p·q)/ r.sub.1 (1+S×10.sup.-6)!}·f.sub.ro (3)

holds.

The processor 12 estimates the correct frequency f_(x) to be measured byinserting the internal reference frequency f_(r), the error correctioninformation S, and the counts into the equations (2), (3).

In accordance with the aforementioned error correction operation thereis no need of previously rendering the OCXO 4 to temperaturecompensation, and a deviation of the frequency f_(x) from the absolutefrequency may be allowed at the normal temperature.

The temperature compensation and the deviation are corrected by theprocessor 12 based on the temperature sensor output and the contents inthe non-volatile memory 10.

Further, also a deviation of the frequency f_(x) with the lapse of timemay be moderated by altering the contents in the non-volatile memory 10without being moderated for the OCXO 4.

In accordance with the first embodiment, a crystal oscillator can bedesigned without intending a varying function of a frequency asdescribed above. Accordingly, any crystal oscillator can be designedwithout requiring expert technical knowledge, and such a crystaloscillator that is very stable with respect to variations with respectto variations with the lapse of time and variations of a power supplyexcepting a temperature characteristic can be obtained with ease.

There is further eliminated the need of a greatly monotonicallydecreasing frequency-temperature characteristic, and there is selectablea cutting angle of a crystal which provides least variations of afrequency in a necessary temperature range, say an equi-ripplecharacteristic, and hence a rate of compensation for such a temperaturecharacteristic, the so-called compressibility is reduced. This alsoensures a stable crystal oscillator.

Further, there is no limitation that a capacitance ratio of the crystaloscillator 15 must be reduced, the limitation being caused by providinga frequency varying function to the OCXO 4. Accordingly, there can beused a crystal oscillator having a higher capacitance ratio but havinglower variations with the lapse of time. The higher capacitance ratiobrings about reduced component sensitivity with respect to a change inany circuit component in the oscillation loop and further brings aboutthe simplification of the oscillation loop. This ensures a very stableoscillator.

It is well known that the capacitance ratio of a crystal oscillator isproportional to a ratio between a plate area and an electrode area.Reduction of the electrode area causes vibration energy to beconcentrated to the center of the plate and hence strain from a supportsystem and the influence of relaxation of the strain with the lapse oftime to be reduced.

Provided the number of overtones is assumed to be n, the capacitanceratio is proportional to the square of n. Provided the number ofovertones is selected to be 3 or 5, sensitivity to a change in theelectrode (relaxation of chemical change or strain) can becorrespondingly reduced.

With the present technology, temperature measurement is achieved veryinexpensively and with high accuracy, say, with a resolution of 0.01° C.A thermostatic oven with temperature stability of 0.001° C. isrealizable using a commonly available thermistor. Stability andreproducibility of the thermistor have been established. An electricallyerasable/writable non-volatile memory (EEPROM) with satisfactorycapacitance is inexpensively available. The apparatus of the presentembodiment is therefore manufactured easily by, persons other than thoseskilled in the art without causing any difficulty of accuracy.

In the following, there will be described a first modified example ofthe first embodiment of the frequency measuring apparatus according tothe present invention with reference to FIG. 6.

The first modified example is to realize the temperature measurement bythe aforementioned temperature sensor 5 inexpensively with low powerconsumption and high accuracy, and is constructed as illustrated in FIG.6 such that the sensor output of the temperature sensor 5 is connectednot with the processor 12 but with the gate circuit 7 for temperaturemeasurement by the first or second counter 8 or 9. Other construction isidentical to that of the embodiment illustrated in FIG. 1.

In the first modified example, the processor 12 activates thetemperature sensor 5 at the time temperature measurement is necessary,and provides a command so as to generate a pulse having a widthcorresponding to temperature. The sensor output of the temperaturesensor 5 is fed to the gate circuit 7. The processor 12 instructs thegate circuit 7 such that the first or second counter 8 or 9 counts theoutput fr of the OCXO 4 only when the temperature sensor output ispossible.

With the construction, high resolution temperature measurement isensured only with slight addition to the circuit of the frequencymeasuring apparatus. More specifically, the first or second counter 8 or9, that is originally to measure frequency, has a sufficient number offigures (e.g., decimal 7 figures). Thus, the high resolution temperaturemeasurement is ensured.

FIG. 7 is a concrete circuit diagram of the temperature sensor 5 in thefirst modified example.

With the temperature sensor 5, highly accurate (e.g., 1/100° C.)temperature measurement is ensured with very little consumed power.

The temperature sensor 5 comprises, as illustrated in FIG. 7, a switch17 connected to power supply voltage, a timer 18 connected to the switch17, a thermistor 19 connected to the switch 17 and the timer 18, and acapacitor 20 connected to the thermistor 19 and the timer 18.

Reproducibility of stability of the thermistor 19 as a temperaturesensitive component has been technically satisfactorily established atpresent. The time 18 uses a CMOS 555 timer that is an industrialstandard. The timer 18 includes therein a resistor for dividing voltagebetween a power supply and an earth terminal, and hence consumes slightpower, less than 1 mW at all times.

For achieving less power consumption, the processor 12 switches thepower supply on only upon the measurement. The pulse width from thetimer 18 is 1.1 times a product of the resistor of the thermistor 19 andthe capacitance 20. No voltage is applied to the thermistor 19 and thetimer 18 at ordinary times to prevent error from being produced owing toself-heating, the error causing a trouble upon the temperaturemeasurement.

In the following, there will be described a second modified example ofthe first embodiment of the frequency measuring apparatus according tothe present invention.

The second modified example is to speed up the estimation of thefrequency f_(x) to be measured based on the error correction informationS and the internal reference frequency f_(r), in which the OCXO 4 ispreviously designed such that the error correction information S issecurely negative within a necessary temperature range.

More specifically, provided the error correction information S isnegative, in an equation, which is obtained by rewriting theaforementioned equation (3), t surely gets 1 or more. ##EQU1## Since tis a value very close to 1, holds, and hence a product of Δt havingrelative small figures may be executed without considering a signthereof.

    f.sub.x = (p·q)/r!*(1+Δt)f.sub.ro           (5)

Δt may be obtained by referring to a table of the non-volatile memory 10based on the temperature sensor output at a necessary time interval. Itis well known that processing time of multiplication in the processor 12is shorter as the number of figures is smaller with no sign comparedwith the case with any sign. Provided Δt is secured to be alwayspositive, it is effective for shortening of the processing time.

It is not difficult to design and manufacture the OCXO 4 such that theerror correction information S is surely negative within a necessarytemperature range. That is, provided a higher capacitance ratiooscillator is used as described previously, component sensitivity of theoscillator to other oscillation circuit components is low and hence theoscillator is prevented from being affected by variations of the circuitcomponents constituting the oscillation loop.

Since a crystal oscillator for use in the present invention ismanufactured at a cutting angle close to the so-called zero temperaturecoefficient, variations of the temperature characteristic thereof arealso reduced. It is therefore not difficult to finely adjust theoscillator with relatively high accuracy, say, at +10 ppm above theobject reference frequency.

In the following, there will be described a packaging (i.e., externalperipheral structure) of the OCXO 4 and the temperature sensor 5 in thefirst embodiment of the frequency measuring apparatus according to thepresent invention with reference to FIG. 8.

It is first supposed that the frequency measuring apparatus according tothe present invention is a module inserted into an extension slot of apersonal computer. Such a module has features: it is thin, it isnonobvious that what module is inserted into an adjacent slot, and therise of internal temperature is greater and an air flow by a fan isexistent.

FIG. 8 is a perspective view of a package structure of the OCXO 4 andthe temperature sensor 5 of a frequency measuring apparatus whichensures accurate frequency correction even under such severe conditions.

A crystal oscillator associated with the present invention has reducedcomponent sensitivity but allows the crystal oscillation loop to gethigh impedance. It is therefore preferable that the OCXO 4 and thetemperature sensor 5 are electrically shielded with a metal container 21which is in turn provided with heat capacity to prevent an internaltemperature change therein from being increased.

Although in FIG. 8 the metal container 21 is depicted so as to coveronly one surface of the printed circuit board 22, it is preferable toconstruct the back surface of the same in the same manner. Use of asurface mounting technique (SMT) ensures that the back surface is formedas one metal plate.

The package is devised such that thermal resistance against heatconduction from the printed circuit board 22 is increased. Morespecifically, a groove 23 is formed around the OCXO 4 and thetemperature sensor 5 excepting a necessary portion to maintainelectrical connection and mechanical strength by making use of a mold orrouting. Hereby, for the OCXO 4 and the temperature sensor 5 thermaltime constant for several minutes is ensured even under theaforementioned severe conditions.

Further, hereby, error due to a difference between the thermal timeconstants of the crystal and the temperature sensitive component whichis conventionally likely to be produced in temperature compensation ofthis type can be moderated and error due to a time difference betweentemperature measurement and frequency measurement following a steeptemperature change can be reduced to be negligibly small. This is alsoeffective to reduce on overhead for the temperature measurement and forobtaining a correction factor for the frequency measurement bylengthening repetition time of the temperature measurement.

Although in the first embodiment the case of the use of the OCXO as thecrystal oscillator was described, it is also applicable to frequencymeasuring apparatuses using a temperature compensation crystaloscillator (TCXO) and other types of crystal oscillators.

In the following, there will be described a second embodiment of thefrequency measuring apparatus according to the present invention.

Although the foregoing first embodiment was applicable also to frequencymeasuring apparatuses using crystal oscillators other than the OCXO, thepresent second embodiment is applicable only to Ocxo which can improveaccuracy by forcing it to maintain minimum point temperature of thefrequency-temperature characteristic of a crystal oscillator.

In the second embodiment, there is different the operation of a portioncomposed of the OCXO 4, the temperature sensor 5, the non-volatilememory 10, and the processor 12 in the first embodiment illustrated inFIG. 1. Accordingly, the second embodiment will be described withreference to FIG. 9 illustrating a block diagram only of the foregoingdifferent portion.

The second embodiment includes an OCXO 26 and a processor 25 asillustrated in FIG. 9. The OCXO 26 is constructed such that a crystaloscillator 15 and an oscillation/output unit 16 both forming anoscillation loop are disposed in a thermostatic oven 14 controlled topredetermined temperature by a thermostatic oven control unit 13 and anon-volatile memory 27 is provided on the thermostatic oven control unit13 as illustrated in FIG. 10. Further, as illustrated in FIG. 9, thethermostatic oven 13 includes a thermostatic oven temperature controlunit 28, a digital/analog converter (D/A) 29, and a micro controller 30.The micro controller 30 reads information stored in the non-volatilememory 27 and transmits it to the thermostatic oven temperature controlunit 28 through the D/A converter 29:

A temperature sensor 31 is provided in the thermostatic oven 14, anoutput from which is supplied to the thermostatic oven temperaturecontrol unit 28 to monitor temperature in the thermostatic oven andcontrol it to arbitrary temperature.

A quartz plate is cut to substantially obtain desired minimum pointtemperature for the purpose of obtaining the minimum point temperatureof the crystal oscillator 15, and a proper thickness electrode supposedto substantially yield a desired oscillation frequency is deposited onthe crystal plate. The temperature in the thermostatic oven is alteredupon adjustment of the OCXO and the oscillation frequencies atrespective temperatures are detected to specify the minimum pointtemperature. More specifically, the processor 25 supplies firsttemperature information to the memory 27, and simultaneously themicrocomputer 30 reads the information and transmits it to thethermostatic oven temperature control unit. 28 through the D/A converter29 whereby the temperature in the thermostatic oven is adjusted to thefirst temperature.

Once the interior of the thermostatic oven reaches the firsttemperature, the processor 25 measures an oscillation output frequencyof the OCXO highly accurately (about 10⁻⁸ for example) and stores it ina memory device included in the processor.

The processor 25 then supplies second temperature information higherthan the first temperature to the memory and likewise assumes thetemperature in the thermostatic oven to be second temperature, andthereafter measures the oscillation output frequency of the OCXO andestimates a difference between the frequency at the first temperatureand the frequency at the second temperature and stores it in the memory.

Identical control is executed in succession at a predeterminedtemperature interval to obtain three or more to frequency-temperaturedata.

The control is executed until a difference between the n-th frequencyf_(n) and the (n-1)-th frequency f_(n-1) becomes negative, i.e., f_(n-1)-f_(n) <0 is attained, and on the basis of the result the minimum pointtemperature of the crystal oscillator 15 of the OCXO is estimated.

Although the minimum point temperature is approximately estimated fromthe resulting 3 or more information, for more accurately detecting theminimum point temperature a plurality of temperatures are searched suchthat the frequency difference f_(n-1) -f_(n) is substantially zero. Morespecifically, in this method the temperature in the thermostatic oven isaltered in succession as described above and a difference betweenoscillation frequencies at successive two temperatures is estimated. Asthe difference becomes smaller, the amount of a change in the nexttemperature is reduced. Hereby, data substantially coincident with afrequency-temperature characteristic curve in the vicinity of theminimum point temperature is obtained and hence detection of accurateminimum point temperature is ensured.

The minimum point temperature information obtained in such a manner isstored in the non-volatile memory 27 following a predetermined format.

Once the minimum point temperature information is stored in the memory27, for the OCXO thereafter the thermostatic oven control unit 13 isactuated following the minimum point temperature information stored inthe memory 27 to control the thermostatic oven such that the interior ofthe same reaches the minimum point temperature and hence the oscillationfrequency is kept unchanged.

Upon adjustment of the OCXO so controlled to the minimum pointtemperature, the oscillation output frequency is measured to obtaindifference information (frequency deviation information) between it andnominal frequency (absolute frequency to be taken as a reference, whichinformation is in turn stored in the non-volatile memory 27. Error ofthe associated frequency is measured with a highly stable highlyaccurate frequency measuring apparatus prepared separately, as a matterof course.

Although basic memory information required in the second embodiment isas described above, provided the minimum point temperature adjustmentfor the thermostatic oven is as usual, information to be stored in thenon-volatile memory is only the minimum point temperature informationand the difference information between the oscillation frequency at thattemperature and the nominal frequency (absolute frequency to be taken asa reference).

A standard frequency and the output frequency of the present oscillatorare compared with each other in a final inspection stage and a result ofthe comparison is stored in the non-volatile memory 27, with itsdeviation expressed in the form of ppm, Hz, or a coefficient (1,00001234for example) or the absolute frequency for example.

With the construction described above, even though the minimum pointtemperature of a crystal oscillator used is varied for each product, itis automatically detected and stored in the non-volatile memory 27whereby the thermostatic oven is kept unchanged in its interior at theminimum point temperature thereof so that a resulting oscillationfrequency is always stably coincident with that at the minimum pointtemperature. Thus, the need of a complicated minimum point temperatureadjustment is eliminated.

In the foregoing OCXO simultaneously with the minimum point temperatureinformation the difference information between the oscillation frequencyat that minimum point temperature and the nominal frequency is outputfrom the nonvolatile memory 27, so that provided in a frequencymeasuring apparatus using the OCXO the OCXO output frequency informationis corrected on the basis of the difference information outputsimultaneously likewise the case, in the first embodiment, absolutelyaccurate frequency information is ensured for insurance of very highaccuracy measurement.

The following information is conveniently stored in the foregoingnon-volatile memory additionally to the above description.

Although a high accuracy measuring apparatus is a system wherecalibration is regularly performed and measurement error is proved to bewithin an allowable limit, the OCXO is likewise an object to becorrected provided it is used as a measuring apparatus.

For this, provided a calibration date and a calibration value are firststored in the non-volatile memory, details of the calibration can beknown by reading them at need.

Secondly, also storage of data concerning a long-term agingcharacteristic (ppm/year) and a short-term aging characteristic(ppm/day) or data concerning the time the frequency of the OCXO isstabilized after power is supplied thereto and the tendency of a changein that time in the memory brings about convenience to users.

More specifically, generally in high accuracy measuring apparatuses anoscillator with required stability (aging characteristic rank) isfrequently mounted thereon through a plug-in system in response todesired measurement accuracy and an aging rank of an oscillator used formeasurement data is sometimes entered, so that these pieces ofinformation are previously stored in the memory, which are in turn readout through a processor disposed on the side of the measuring apparatusconveniently for display and printing. Particularly, agingcharacteristic information ensures a clue to know the number ofeffective figures of an oscillation frequency when the OCXO is used sothat the information is effectually used on clarifying measurementaccuracy.

Further, additionally to the above description lot numbers andmanufacturing numbers of oscillators and Ocxo and dates of manufactureof the formers may be stored at need in the non-volatile memory.

Furthermore, in accordance with the second embodiment, many processesconcerning a conventional OCXO are reduced and the low cost is ensured,as described below.

Since there is no need of adjusting and varying an associated frequencyusing an electronic circuit, there is eliminated a limitation to acapacitance ratio of a crystal oscillator, i.e., a ratio of parallelcapacitance of the crystal oscillator and capacitance of an equivalentseries resonance circuit. In contrast, although for making variable anoscillation frequency in an oscillation circuit it is necessary toreduce a capacitance ratio, the limitation is eliminated and thecapacitance ratio can be selected to be larger.

It is well known that a capacitance ratio of a crystal oscillator isproportional to a ratio of a plate area and an electrode area. Providedthe electrode area is reduced, vibration energy is more concentrated atthe center of the plate to reduce strain from a support system and theinfluence of a change with the lapse of time in the strain. Since thecapacitance ratio is also proportional to the square of the number ofovertones, provided a higher order overtone is selected, sensitivity toa change in the electrode (relaxation of a chemical change and strain)can correspondingly be reduced.

Further, since a frequency adjustment is done to moderate themanufacturing deviation of a crystal oscillator by altering the constantof a reactance network in the oscillation loop, there is eliminatedinconvenience of a required negative resistance on the side of anoscillation circuit being sharply varied, so that variations ofoscillation loop conditions are eliminated to ensure a stable outputwith predetermined oscillation redundancy at all times.

Furthermore, effect of the OCXO on circuit construction is also greater.

More specifically, there is eliminated the need of a frequencyadjustment unit and a frequency varying unit in the oscillation loop toimprove the reliability of the circuit and eliminate the possibility ofa change in the output frequency of the oscillator resulted in followinga change with the lapse of time when circuit components are heated tohigh temperature. Particularly, a glass trimmer condenser for mechanicalvariation, a varactor diode for electronic variation, a potentiometer,and a reference voltage source and the like are generally used forprovision of a frequency variation function, and reliability of theseparts is lower than general passive parts so that a reliabilityimprovement by the elimination thereof will be easily understood.

More specifically, a network of a frequency adjustment unit and afrequency varying unit in a conventional OCXO is very complicated and agreater volume component such as a glass trimmer condenser is used.

Accordingly, elimination of these components obviously reduces the sizeof the thermostatic oven and simultaneously reduces a consumed current.

Furthermore, each of the aforementioned non-volatile memory,microcomputer, and D/A converter is easily commercially available in theform of a single chip of a miniature package, and hence an integratedcircuit unifying them is also easily available so that any influence onthe cost, volume, and reliability can be restricted to a degree nottroubled actually.

Additionally, use of a microcomputer also for the thermostatic ovencontrol ensures higher degree nonlinear control than mere analogproportional control such as a conventional OCXO. For example, there isalso ensured such a temperature change as minimizing a thermal transientphenomenon possessed by an oscillator.

Although in the above embodiments there was described the case where anOCXO is used as a crystal oscillator, the present invention is likewiseapplicable to frequency measuring apparatuses using a temperaturecompensation crystal oscillator (TCXO) and crystal oscillators of othertypes without being limited to the above embodiments.

What is claimed is:
 1. A frequency measuring apparatus for measuring thefrequency of an input signal to be measured with high accuracycomprising:a crystal oscillator for generating an internal referencefrequency; a temperature sensor provided on said crystal oscillator;writable memory means; estimation control means for storing in saidmemory means a correspondence relation between an absolute frequency tobe taken as a reference at a predetermined temperature measured by saidtemperature sensor and the internal reference frequency from saidcrystal oscillator as error correction information, and estimating thefrequency of said input signal to be measured in conformity with theinternal reference frequency corrected by said error correctioninformation; and a display means for displaying a frequency valueobtained by said estimation control means.
 2. A frequency measuringapparatus according to claim 1 wherein said crystal oscillator comprisesan oven-controlled crystal oscillator (OCXO) using a thermostatic oven,and said temperature sensor measures temperature in the thermostaticoven.
 3. A frequency measuring apparatus according to claim 1 whereinsaid crystal oscillator is constructed such that an output frequency ofsaid crystal oscillator is surely higher than an object referencefrequency within an operation temperature range.
 4. A frequencymeasuring apparatus according to claim 1 wherein said crystal oscillatorand said temperature sensor are formed on the same printed circuit boardas that of other electronic circuits, and a groove is provided aroundsaid crystal oscillator and said temperature sensor such that saidcrystal oscillator and said temperature sensor provide thermally higherresistance, and further said crystal oscillator and said temperaturesensor are surrounded with metal.
 5. A frequency measuring method ofmeasuring the frequency of an input signal to be measured with highaccuracy comprising the steps of:measuring a correspondence relationbetween an internal reference frequency and a reference absolutefrequency at a plurality of temperature measurement points of a crystaloscillator for generating said internal reference frequency; storingsaid measured correspondence relation in memory means as errorcorrection information; measuring, when said signal to be measured isinput, the temperature of said crystal oscillator at that time; readingsaid error correction information at said measured temperature from saidmemory means; correcting said internal reference frequency based on saidread error correction information and estimating the frequency of theinput signal to be measured in conformity with the corrected internalreference frequency; and displaying a value of frequency of said inputsignal which is obtained in said step of estimating.
 6. A frequencymeasuring apparatus for measuring the frequency of an input signal to bemeasured with high accuracy comprising:an oven-controlled crystaloscillator (OCXO) using a thermostatic oven to generate an internalreference frequency; a temperature sensor for measuring temperature inthe thermostatic oven of said OCXO; writable memory means; estimationcontrol means for executing various estimation control; and a displaymeans for displaying a frequency value obtained by said estimationcontrol means, and the estimation control means executing a step forobtaining an error correction information and a step for measurement;and said step for obtaining an error correction information comprisingthe steps of:counting for a predetermined period of time an internalreference frequency and an absolute frequency, which is inputted fromoutside, at a plurality of temperature measuring points of saidthermostatic oven measured by said temperature sensor, and determining adifference or a ratio of a result of said counting, storing in saidmemory means a minimum point temperature of crystal plate of said OCXO,said minimum point temperature being obtained from a value determined insaid step of determining, setting a temperature of said thermostaticoven of said OCXO at said minimum point temperature, and storing in saidmemory means a correspondence relation as error correction information,said correspondence relation being between an absolute frequency to betaken as a reference and an internal reference frequency near saidminimum point temperature and from said OCXO and obtained from saidvalue determined in said step of determining, and said step ofmeasurement comprising the steps of:estimating, when said signal to bemeasured is input, a frequency of said signal to be measured inconformity with said internal reference frequency which is corrected bysaid error correction information, and displaying a result of said stepof estimating on said display means.
 7. A frequency measuring apparatusaccording to claim 6 wherein said OCXO includes a thermostatic ovencontrol unit for controlling the temperature in the thermostatic oven tothe minimum point temperature in conformity with the minimum pointtemperature information from said memory means.
 8. A frequency measuringapparatus according to claim 6 wherein said estimation control meanscontrols said memory means such that said memory means stores an agingproperty or identification information of said OCXO therein.
 9. Afrequency measuring method of measuring the frequency of an input signalto be measured with high accuracy comprising the steps of:counting for apredetermined period of time an internal reference frequency and anabsolute frequency, which is inputted from outside, at a plurality oftemperature measurement points of an oven-controlled crystal oscillator(OCXO) using a thermostatic oven for generating said internal referencefrequency, and determining a difference or a ratio of a result of saidcounting; estimating minimum point temperature from information of theinternal reference frequency at said plurality of temperaturemeasurement points; storing said estimated minimum point temperature inmemory means; setting a temperature in the thermostatic oven of saidOCXO at said minimum point temperature in conformity with the minimumpoint temperature from said memory means; measuring a correspondencerelation between an absolute frequency to be taken as a reference atsaid minimum point temperature and the internal reference frequency fromsaid OCXO near said minimum point temperature; storing said measuredcorrespondence relation as error correction information; reading saiderror correction information from said memory means upon said inputsignal to be measured being input; correcting said internal referencefrequency based on said read error correction information and estimatingthe frequency of said input signal in conformity with the correctedinternal reference frequency; and displaying a frequency value of saidsignal to be measured, said frequency value being obtained in said stepof estimating.